Originally offshore oil and gas wells were completed on platforms resting on the ocean bottom, or were completed on the bottom with surface production facilities installed on the platform. Oil and gas produced at such platforms were either collected by tankers or by flowlines laid on the bottom. To the extent that underwater work was required, it was performed by divers, submarines, or simple manipulations from the surface.
More recently it has become necessary to drill oil and gas wells in water which is too deep or dangerous for convenient underwater operations by divers or for platforms to stand on the bottom. In such situations new oil field drilling and production techniques located on the ocean floor have been developed including multi-well subsea production manifolds and satellite well production systems. Multi-well subsea production manifolds collect oil and gas from multiple production units in the manifold for transportation to the surface. Satellite well production systems include multiple subsea satellite Christmas tree assemblies which transport the produced oil or gas to a central production location for transport to the surface. Thus, it has become necessary to devise methods of connecting such multiwell subsea production manifolds and satellite well production systems at the ocean floor with flowlines, hydraulic control lines and electrical cables extending to the surface without the use of divers or any permanent surface structure adjacent the underwater production system. In the case of multi-well subsea production manifolds, often there are a plurality of hub assemblies disposed on the manifold structure for connection to flowlines, hydraulic control lines and electrical cables. In the case of a satellite well production system, the production tree will also include numerous hub assemblies for connection to flowlines, hydraulic control lines and electrical cables extending to a central manifold or directly to the surface. A better understanding of the problems presented by such systems may be obtained by reference to the January, 1978 issue of Offshore Services magazine, published by Spearhead Publications Limited, at pages 26-51.
In making up a remote flowline connection there are two basic modes, the layaway mode and the pull-in connection mode. In the layaway mode a flowline bundle containing an outboard hub mounted on the free end of the flowline, complete with production lines, electrical cable, and multiple hydraulic lines, is passed from the lay barge to the drilling vessel and lowered on the guide wires until it lands on the guide posts and is locked in place. The hub is to be properly aligned vertically, horizontally, and rotationally with a mating inboard hub on the production unit where the inboard hub is attached to the lines on the ocean floor. Once aligned, the hubs are connected together, and laying of the pipe bundle proceeds.
When using the pull-in connection mode, the flowline bundle with outboard hub and lines are mounted on a sled. Provision is made on the production unit for hauling the sled to the production unit. Continued hauling brings the sled to a ramp on the production unit where the sled is drawn up the ramp and landed in the correct position for mating with the inboard hub on the production unit. The hub sled is then locked into place. The Christmas tree is run and mated with the wellhead and a collet connector connects the tree to the wellhead. Making up the connection of the mating hubs then follows the procedure of the layaway mode.
Thus, in utilizing the pull-in connection mode for connecting by remote operation, flowlines and hydraulic lines for underwater production units, it is essential to insure that the flowline bundles are properly aligned vertically, horizontally, and rotationally before they are latched onto the structure of the underwater production unit. Apparatus which has previously been designed for this purpose has required the application of high pulling loads from the surface to guide the flowline bundle into and through mechanical alignment devices for proper alignment and positioning. These high loads frequently lead to mechanical failures and the remoteness of the point of generation of these loads from the point application of such loads does not permit the high degree of control necessary for final alignment operation.
Further, prior art devices have been limited in their ability to align the bundle of flowlines, hydraulic lines and electrical cables and have been unable to adjust the aligment in all directions.
Previous designs have generally employed mechanical funnels, pivotable or fixed, for alignment purposes. The incoming flowline had to be pulled, with great force, into the funnel from aboard the drilling vessel. Quite frequently the load required to be applied proved to be both insufficient to overcome mechanical obstacles and too great for the cable or the guide post of the underwater production unit resulting in breakage of the cable or bending of the posts.
Once the lines are properly positioned, various devices have been proposed for achieving the underwater connection as shown, for example, in U.S. Pat. No. 3,968,382 to Baugh, U.S. Pat. No. 4,019,334 to Sinclair et al and U.S. Pat. No. 4,086,778 to Latham et al.
The apparatus and method of the present invention overcomes the deficiencies of the prior art by providing, among other things, a hydraulic cable puller which is located on the underwater production unit at the ocean floor, a bullnose latch and bullnose for determining vertical positioning and a hydraulic alignment device for final alignment of the flowline bundle. The present invention is also equipped for severing the steel cable used for pulling the flowline bundle in the case of an emergency or at the end of a successful pull-in operation. The hydraulic alignment device overcomes the deficiencies of those prior art devices which utilized a helical surface guide inside the alignment funnel which had mechanical resistance that had to be overcome by the help of an increasing pulling force from above the sea level. The design of the present invention is such that all hydraulic equipment required for pull-in, alignment and severing the cable can then be removed to the surface, no portion of it being required to remain on bottom. Further objects and advantages of the present invention will be apparent from the following description.